late 15c., "of the nature of an agreement," from Late Latin conventionalis "pertaining to convention or agreement," from Latin conventionem (see convention). Meaning "of the nature of a convention" is from 1812, now rare; "established by social convention" is from 1761; that of "following tradition" is from 1831; that of "non-nuclear" is from 1955. Realted: Conventionality; conventionally.